Dr. Y.K. Bae demonstrated a Photonic Laser Thruster (PLT) built from off-the-shelf optical components and a YAG gain medium, and the maximum amplified photon thrust achieved was 35 µN for a laser output of 1.7 W with the use of a HR mirror with a 0.99967 reflectance. This performance corresponds to an apparent photon thrust amplification factor of ~3,000. More importantly, in the experimental demonstration, the author accidentally discovered that the PLT cavity is highly stable against the mirror motion and misalignment unlike passive optical cavities. In fact, in the demonstration experiment by Dr. Bae, the full resonance mode of the PLT was maintained even when one of the HR mirror was held by a hand. In a more systematic experiment, the PLT cavity was demonstrated to be stable against tilting, vibration and motion of mirrors. Subsequent theoretical analysis by the author showed that PLT can indeed be used for propulsion applications, and proposed Photonic Laser Propulsion (PLP), the propulsion with PLT. The reason for the observed stability results from that in the active optical cavities for PLT and PLP the laser gain medium dynamically adapts to the changes in the cavity parameters, such as mirror motion, vibration and tilting, which does not exist in the passive optical cavities.

A four-phased evolutionary developmental pathway of the Photonic Railway towards interstellar manned roundtrip travel is proposed:
1) Development of PLTs for satellites and NEO manipulation,

The first phase in the developmental pathway towards interstellar roundtrip manned flight is maturing PLT technologies and systematic scaling up of its power and operation distance capabilities. PLT is predicted to meet the needs of the next generation of space industry market by enabling a wide range of innovative space applications near the earth. In this phase, which is predicted to evolve over 5–30 year time frame, PLT would be capable of providing thrusts in the range of 1 mN–1 kN, which requires the operation power of 100 W–100 MW. The solar panel based space power currently can provide electrical powers up to 100 kW, therefore, the PLT capable of providing thrusts up to 1 N can be readily implemented in the near future.

Traditionally, Direct Energy (DE), especially High Energy Laser (HEL), has mainly been considered for beaming energy or power, however, it can be exploited for beaming momentum as well. Dr. Y. K . Bae presents an innovative spacecraft maneuvering architecture, DE Momentum Beaming (DEMB), in which momentum is beamed between two spacecraft platforms via the pressure of circulating photons between them with the use of recently developed Photonic Laser Thruster (PLT). Many advanced DoD in-space missions need a wider range of dynamic spacecraft maneuvers than formation flying. Conventional spacecraft maneuvering is performed by momentum applied to a single vehicle by exhausting fuel in forms of plumb or ions, which limits lifetime and delta-V capability. Through momentum beaming, DEMB will drastically reduce the fuel consumption or separate the highly valuable mission vehicle from a lower-cost, replaceable resource vehicle (similar to aerial refueling) to lower the life-cycle cost significantly in a wide range of missions. Therefore, DEMB is projected to enable a wide range of next-generation DoD missions in space and provide ways to enhance existing mission architectures. Exemplary missions that can be enabled by DEMB include that involve orbit-raising or escape, drag compensation, and rendezvous and docking. In addition, the specific DE technologies required for developing DEMB are also discussed.

Bae proposes a propellant free, thus contamination free, method that enables ultrahigh precision satellite formation flying with intersatellite distance accuracy of nm (10^-9 m) at maximum estimated distances in the order of tens of km. The method is based on ultrahigh precision CW intracavity photon thrusters and tethers. The pushing-out force of the intracavity photon thruster and the pulling-in force of the tether tension between satellites form the basic force structure to stabilize crystalline-like structures of satellites and/or spacecrafts with a relative distance accuracy better than nm. The thrust of the photons can be amplified by up to tens of thousand times by bouncing them between two mirrors located separately on pairing satellites. For example, a 10 W photon thruster, suitable for micro-satellite applications, is theoretically capable of providing thrusts up to mN, and its weight and power consumption are estimated to be several kgs and tens of W, respectively. The dual usage of photon thruster as a precision laser source for the interferometric ranging system further simplifies the system architecture and minimizes the weight and power consumption. The present method does not require propellant, thus provides significant propulsion system mass savings, and is free from propellant exhaust contamination, ideal for missions that require large apertures composed of highly sensitive sensors. The system can be readily scaled down for the nano- and pico-satellite applications. Formation flying of clusters of micro-, nano- and pico-satellites has been recognized to be more affordable, robust and versatile than building a large monolithic satellite in implementing next generation space missions requiring large apertures or large sample collection areas and sophisticated earth imaging/monitoring.

One of the factors that limit the maximum obtainable velocity of the accelerating mirror and its accommodating spacecraft is limited by the Doppler shift of the bouncing photons. Doppler shift effect on the active resonant cavity behavior is an extremely complicated issue, which is beyond the scope of the current paper. Eventually, this aspect should be studied with computer optical simulation. Optical gain in the laser cavity can only occur for a finite range of optical frequencies. The gain bandwidth is basically the width of this frequency range. For example, the gain bandwidth of the YAG laser system with the laser wavelength in the order of 1,000 nm is in the order of 0.6 nm, which is ~ 0.06 % of the wavelength. For an order of magnitude estimation, we assume that PLT utilizing the YAG laser system will be limited by the gain bandwidth to the first order, then, theoretical maximum spacecraft velocity is ~1.8 x 10^5m/sec (180 km/sec) that is 0.06 % of the light velocity, c=3×10^8m/sec. To overcome this redshift limitation, PLT, at high operation velocities, should employ wide bandwidth lasers.

Traditionally, the intracavity laser arrangement required for PLT operation had been operated in relatively short cavities less than 10 m long. Therefore, there has been a concern that the action distance of PLT may not be more than tens of meters. However, recently, Bohn of the German Aerospace Center (DLR) reported that the German company Rheinmetall Defense demonstrated a 1-km long laser resonator similar to the PLT optical resonator in 1994-1995 with the use of a telescopic arrangement in the optical cavity, and that such long laser resonators can be scalable to 100 km with the usage of optics in the diameter of 70 cm. These successful demonstrations promise that PLT can be operated beyond distances in the order of 100 km. Further studies should be performed whether PLT can be used for interstellar scales, but so far there is no show stopper on this issue.

One of key technological issues in implementing PLT is in the intracavity laser beam aiming, aligning, and tracking. In ABL (Anti-ballistic Laser), the aiming, alignment, and tracking of the main laser rely on the scattered beam of the beacon laser (also diode pumped lasers at power level of a few kW.) Similar to this, a small laser (power level of a few watts) in the mission vehicle can be used as a beacon laser. It seems that the aiming, aligning, and tracking system can be scaled to interstellar distances.